1. Technical Field
The present invention relates to an electrophoretic device, provided with a dispersal system including electrophoretic particles, a driving method thereof, and an electronic apparatus that utilizes the device.
2. Related Art
A phenomenon called electrophoresis, in which electrophoretic particles are moved by a coulomb's power, when an electric field is applied to a dispersal system, and the electrophoretic particles are distributed in a solution, is known, and electrophoretic devices, which utilize that phenomenon have been developed. Such electrophoretic devices are disclosed in literatures such as JP-A-2002-116733, JP-A-2003-140199, JP-A-2004-004714, and JP-A-2004-101746. These are examples of the related art. However, common electrophoretic devices involve a problem of image quality, leaving much room for improvement. Specific examples related to this problem will be described hereafter.
FIG. 12 is a diagram that describes an example structure of circuitry for an active-matrix electrophoretic device. The electrophoretic device shown in the diagram has a plurality of scanning lines and a plurality of data lines that are arranged orthogonally to each other, the cross points of which have the electrophoretic elements installed on them. A dispersal system is laid between a common electrode and a pixel electrode that are arranged to face each other, constituting the electrophoretic element. A current is supplied to each electrophoretic element by a transistor connected to the scanning line and the data line.
FIGS. 13A through 13C are wave pattern diagrams that describe the common method for driving the electrophoretic device with the structure shown in FIG. 12. In the driving method shown in FIG. 13, a reset. period that resets all the pixels to be displayed as white is provided, prior to an image signal import period. During this reset period, a low power source potential Vss (for instance, 0V) is applied to the pixel electrodes of the entire pixels, and a high power source potential Vdd (for instance, +10V) is applied as a potential Vcom (a common potential) of the common electrode. Thereafter, in the subsequent image signal import period, the low power source potential Vss is applied as the common potential Vcom, and potentials corresponding to the content of the display image is applied to each pixel electrode via each data line.
FIGS. 14A, . . . 14C through 17A, . . . 17C are drawings that schematically describe behavior of electrophoretic particles in a spatial distribution, driven with the common driving method shown in FIGS. 13A through 13C. In FIGS. 14A, . . . 14C through 17A, . . . 17C, the behavior of particles of the electrophoretic device with a two-particle system, where the particles shown in white (white particles) are charged with a negative potential and the particles shown in black (black particles) are charged with a positive potential, is shown.
The behavior of the electrophoretic particles at a pixel (1,1) where both data line signal X1 and the scanning line signal Y1 are supplied, and where, for instance, the previous screen is displayed as white, and the next screen is displayed as black, is shown in FIGS. 14A through 14C. In the previous screen, as shown in FIG. 14A, the potential Vss is applied as the common potential Vcom to the common electrode, and a potential VL (approximately 0V) is applied to the pixel electrode; thereby the pixel is displayed as white (to be more precise, a grayish white). In the reset period, as shown in FIG. 14B, the potential Vdd is applied as the common potential Vcom, and the potential Vss is applied to the, pixel electrode; thereby the pixel is displayed as white (to be more precise, a strong white), as part of the reset operation. In the next screen, as shown in FIG. 14C, the potential Vss is applied as the common potential Vcom, and the potential Vdd is applied to the pixel electrode; thereby the pixel is displayed as black (to be more precise, a grayish black). Here, since the pixel (1,1) is displayed as strong white during the reset period immediately beforehand, the electrophoretic particles migrate insufficiently; therefore it involves the problem that a subsequent display of black is not black enough.
The behavior of the electrophoretic particles at a pixel (1,2) where both data line signal X1 and the scanning line signal Y2 are supplied, and where the previous screen as well as the next screen are displayed as white, is shown in FIGS. 15A through 15C. In the previous screen, as shown in FIG. 15A, the potential Vss is applied as the common potential Vcom to the common electrode, and a potential VL (approximately 0V) is applied to the pixel electrode; thereby the pixel is displayed as white (to be more precise, a grayish white). In the reset period, as shown in FIG. 15B, the potential Vdd is applied as the common potential Vcom, and the potential Vss is applied to the pixel electrode; thereby the pixel is displayed as white (to be more precise, a strong white), as part of the reset operation. In the next screen, as shown in FIG. 15C, the potential Vss is applied as the common potential Vcom, and the potential Vdd is applied to the pixel electrode; thereby the pixel is displayed as white. Here, the migration of the electrophoretic particles exceeds the necessary, to the extent that the pixel displayed in white is actually a strong white, which causes a relative difference in the brightness from the other pixels, hence causing a disadvantage of the visual afterimage. Moreover, if the pixel being displayed as white further persists, the particles become fixed, white ones to the common electrode side and the black ones to the pixel electrode side. Hence, when the pixel is to be displayed in black, the migration of the particles are less likely to occur, causing the pixel not to be displayed as a desired black. Further, since there is no potential difference between the electrodes when white is displayed, the particles gradually diffuse, causing the white display to turn gray.
The behavior of the electrophoretic particles at a pixel (2,1) where both data line signal X2 and the scanning line signal Y1 are supplied, and where the previous screen is displayed as black, and the next screen is displayed as white, is shown in FIGS. 16A through 16C. In the previous screen, as shown in FIG. 16A, the potential Vss is applied as the common potential Vcom to the common electrode, and a potential VH (approximately 8V) is applied to the pixel electrode; thereby the pixel is displayed as black (to be more precise, a whitish black). In the reset period, as shown in FIG. 16B, the potential Vdd is applied as the common potential Vcom, and the potential Vss is applied to the pixel electrode; thereby the pixel is displayed as white (to be more precise, a grayish white), as part of the reset operation. In the next screen, as shown in FIG. 16C, the potential Vss is applied as the common potential Vcom, and the potential Vdd is applied to the pixel electrode; thereby the pixel is displayed as white. Here, the migration of the electrophoretic particles is less than is necessary, to the extent that the display of the next screen as white actually turns out to be a blackish white, which causes a relative difference in the brightness from the other pixels, hence causing an unfavorable condition of a visual afterimage. Specifically, there is a difference in the level of whiteness from the above-mentioned pixel (1,2).
The behavior of the electrophoretic particles at a pixel (2,2) where both data line signal X2 and the scanning line signal Y2 are supplied, and where the previous screen as well as the next screen is displayed as black, is shown in FIGS. 17A through 17C. In the previous screen, as shown in FIG. 17A, the potential Vss is applied as the common potential Vcom to the common electrode, and a potential VH (approximately 8V) is applied to the pixel electrode; thereby the pixel is displayed as black (to be more precise, a whitish black). In the reset period, as shown in FIG. 17B, the potential Vdd is applied as the common potential Vcom, and the potential Vss is applied to the pixel electrode; thereby the pixel is displayed as white (to be more precise, a grayish white), as part of the reset operation. In the next screen, as shown in FIG. 17C, the potential Vss is applied as the common potential Vcom, and the potential Vdd is applied to the pixel electrode; thereby the pixel is displayed as black. Here, since the electrophoretic particles migrate sufficiently, the display of the next screen as black has an appropriate brightness. However, an unfavorable condition, in which the level of blackness is different compared to the aforementioned pixel (1,1), occurs.
As described, there are various unfavorable conditions existing in the common driving method, and it has been difficult to improve the image quality of the electrophoretic device.